1. Field of the Invention
The present invention relates generally to the field of optical recording media and in particular to the systems and methods for erasing data present on light-responsive optical disks.
2. Description of the Background Art
Optical storage media have become well-known to the public in the form of compact disks for audio and somewhat larger laser disks for combined audio and video. Both types of disks are similar in structure, including a reflective surface protected by a clear plastic substrate layer. Small pits formed in the reflective surface represent data in binary form, and the disks are read by a player which rotates the disk and focuses a laser beam through the substrate layer onto the reflective surface. The presence of a pit increases the optical path of the laser beam by an amount equivalent to a half-wavelength, thereby producing destructive interference when combined with other (non-shifted) reflected beams. The presence of data thus takes the form of a drop in intensity of the reflected light detected by the player.
While compact disks have been enormously successful, presently available commercial disks allow only the playback of pre-recorded information and are not suitable for recording and reuse. To overcome this limitation, various systems for providing recordable and reusable optical storage media have been proposed.
Of particular interest to the present invention are the media and recording systems of the type described in Feyrer U.S. Pat. No. 4,719,615 and copending application Ser. Nos. 152,690; 294,723; 57,377; 357,504; 357,506; 414,044; and 414,041; the disclosures of which are incorporated herein by reference.
The medium disclosed in Feyrer et al. includes a lower expansion layer of a rubbery material which expands when heated. The expansion layer is coupled to an upper retention layer which is glassy at ambient temperature but which becomes rubbery when heated. Both layers are supported on a rigid substrate, and the expansion and retention layers each contain dyes for light absorption at different wavelengths. Data are recorded by directing laser light onto the medium to heat and expand the expansion layer away from the substrate, thus forming a protrusion or bump extending into the retention layer. As the expansion layer expands, the retention layer is heated above its glass transition temperature so that it can deform to accommodate the bump. After the beam is turned off, the retention layer cools to its glassy state before the bump contracts, thereby fixing the bump in the medium.
Reading or playback of the data is then achieved by low intensity "read" laser beam which is focused on the partially reflecting interface between the retention layer and the surrounding air. When the beam encounters the bump, some of the reflected light is scattered, while other portions of the reflected light destructively interfere with the reflected light from flat areas. The resulting drop in intensity is detected by the player.
Data thus recorded on the optical media can be erased using a second laser beam at an "erase" wavelength which is preferentially absorbed by the retention layer but not the expansion layer. The erase beam heats the retention layer to a rubbery state where its viscoelastic forces (and typically in combination with those of the expansion layer) return it to its original flat configuration. While the optical media described in the copending patent applications vary in the details of construction, they all generally allow for writing data with a laser beam at a first wavelength and, optionally, erasing the data with a laser beam operating at a second wavelength.
While most proposed optical disk recording systems allow for erasure using a laser beam which tracks the recorded data points, such systems have a number of disadvantages. First, the need to pass the erase laser beam sequentially over the recorded data tracks is time consuming and will be a significant limitation when erasure is being performed without simultaneous re-recording of data. Second, the use of a laser beam to erase data on an optical disk is constrained by the need to provide sufficient energy transfer to relax the retention layer without overheating either the retention or expansion layers. Because of minor variations in the material characteristics, the use of a laser beam to erase can result in minor variations in the flatness which is ultimately achieved in the retention layer. Finally, use of a laser beam for erasure increases the likelihood of non-uniform thermal effects in the retention and expansion layers. Over time, such non-uniformities can result indegradation of the optical media.
For these reasons, it would be desirable to provide systems and methods for optical disk erasure which do not require tracking of a laser beam sequentially over the disk. In particular, such systems and methods should allow for bulk erasure of the disk in a very short time, preferably within seconds or less. The erasure achieved should be complete with no bumps or protrusions remaining on the media and result in uniform thermal effects in order to assure that all portions of the disk are treated under substantially identical conditions.